The present invention relates to an X-ray CT (computerized tomograph) that obtains an in-body tomographic image of a patient with the use of an X-ray. More particularly, it relates to an X-ray CT that, when continuously measuring almost the same cross section, suppresses the X-ray irradiation toward a region outside a set concerned region up to the smallest possible degree, thereby making it possible to reduce an X-ray exposure dose toward the patient, an operator or a specific tissue.
The X-ray CT has already been used widely in fields such as medical care. For example, in recent years, when endermically executing biopsy of a nidus or the treatment thereof, the X-ray CT is used as the guide to a puncture. Executing, in this way, the biopsy of a nidus or the treatment thereof under the guide by the X-ray CT is now considered to be an effective and helpful method, since this method can be expected not only to shorten an operation time but also to enhance accuracy of the operation.
By the way, for guidance by the X-ray CT, there exist two methods. One method is a method in which the puncture and the CT scanning are repeated alternately and intermittently, while confirming data such as a position of tip of a puncture needle. The other method is a method in which the CT scanning is performed continuously and the image is sequentially displayed so that the position of tip of the puncture needle can be confirmed immediately (CT fluoroscopy). In particular, the latter method permits the tomographic image to be obtained on a real time basis, thus bringing about an advantage of shortening the operation time even further.
However, in the methods where the CT scanning is executed intermittently or continuously as are described above, an increase in the X-ray exposure dose, which is caused by the intermittent or the continuous CT scanning, has become a problem to the patient or the operator. The X-ray exposure dose is determined by the X-ray irradiation dose and slice width set by a slice collimator. In order to reduce the exposure dose, it is sufficient to decrease the irradiation dose just by decreasing a tube electric current passing through an X-ray tube. The decrease in the irradiation dose (mAs=mAxc3x97sec), however, means an increase in the noise by X-ray fluctuation. This, accordingly, has resulted in a problem that picture quality of the tomographic image has been deteriorated exceedingly.
Then, in view of the above-mentioned problems in the conventional techniques, it is an object of the present invention to provide an X-ray CT that is capable of reducing the X-ray exposure dose toward the patient or the operator without causing the deterioration of picture quality of the obtained tomographic image even if the X-ray photography is executed continuously or intermittently at the time of operations such as the above-described puncture based on guidance by the X-ray CT.
According to the present invention, in order to accomplish the above-mentioned object, an X-ray CT is provided that rotates an X-ray source continuously to measure projection data of a subject continuously over a plurality of times and, based on the projection data, reconstructs a tomographic image of the subject to sequentially display it on a display, including a concerned region setting unit that sets, within the subject, an irradiation range of an X-ray emitted from the X-ray source, and an X-ray shielding apparatus, i.e. a channel collimator, that, when rotating the X-ray source to measure the projection data, restricts an irradiation range of a fan beam so as to suppress X-ray irradiation outside of the concerned region set by the concerned region setting unit.
Also, in the present invention, there is provided an image processor that, when a measurement is performed with the X-ray fan beam converged on the concerned region, makes it possible to reconstruct a tomographic image in a region with the use of measurement data obtained previously. The region exists outside of the set concerned region, and the channel collimator suppresses the X-ray irradiation toward the region. The image processor also displays the tomographic image in the region.
Further, in the present invention, in the course of said plural times of continuous measurements, the channel collimator is so controlled as to obtain projection data acquired by normal photography (global measurement scanning) or projection data in a region that is wider as compared with the concerned region, the normal photography being not restricted to the X-ray irradiation region set by the concerned region setting unit.
Moreover, in the present invention, the projection data are measured by irradiating the subject with X-rays while gradually reducing or expanding the X-ray irradiation range between the concerned region set by the concerned region setting unit and the global measurement scanning range.
Furthermore, in the present invention, there is provided an image processor that is capable of reconstructing projection data in a region outside of the concerned region through extrapolation from scanned data obtained by irradiating only the concerned region with X-rays.
In addition, in the present invention, the concerned region setting unit is configured to display on the display a boundary between the inside and the outside of the set concerned region.
Namely, in the present invention, the concerned region setting unit and the channel collimator make it possible to execute the photography in such a manner that the X-ray irradiation toward a region other than the concerned region is suppressed up to the smallest possible degree. Moreover, an image in the region outside of the concerned region is reconstructed by fitting and embedding previously measured data and so on.
As is obvious from the above-described detailed explanation, according to the X-ray CT in the present invention, in operations such as re-inspection and a CT fluoroscopic photography, it is possible to reduce ineffective and needless X-ray exposure by employing local measurement scanning. In the local measurement scanning, only a range obtained by restricting in advance a portion to be photographed (a concerned region) is irradiated with X-rays. Also, as to data in a region other than the concerned region, embedding of data that are previous in time makes it possible to reconstruct an image with fewer artifacts. Namely, these characteristics make it possible to obtain, with a low X-ray exposure dose and concerning the image outside of the concerned region as well, a precise and high picture quality image that has a relatively small difference in time.
In addition, the present invention makes it possible to reduce the X-ray exposure dose toward a subject or the operator without decreasing the tube electric current. This characteristic permits a high accuracy diagnosis or operation to be performed without deteriorating picture quality of the image. Also, regarding the boundary between the inside and the outside of the concerned region, the boundary has been indicated clearly on the image. This is a measure to be taken to prevent a misdiagnosis.